I have a movie clip that i want to move forward so many
pixels with each click of a button. I can move it to a certain x/y
co-ordinate, but i want it to move forward each time you click on
the same button, by so many pixels.

To just move it by steps, simply add or subtract from its
current position.
You say you are moving main, so I'm not sure what the targetx
and targety
properties are - but you can just use movieClip._x to get the
x pos and
movieClip._y to get the y pos. So:

on(press){
this._parent.main._x += 10;
this._parent.main._y += 10;
}

The main._x += 10 is a shorthand for: main._x = main._x + 10
Each time you press the clip moves down and right...